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How can I tell if an aircraft (N-number) is already equipped with an ADS-B transponder?
Is there any database that contains this information?
This question asked how many aircraft already have one. Here I am asking how can I get which contain it.

Can you explain what you need this for? Do you need  to know the equipage for all aircraft on the US register or only a subset? I.e. all aircraft or only for aircraft that fly over your backyard.

In the US there's no requirement to register with any authority the type of transponder that's installed. An aircraft owner is free to swap out the transponder without telling anyone.

If it's onesy twosy, you can try searching flight logs of specific aircraft to see if ADS-B output is recorded.  Here is an example: track log showing ADS-B returns
There is not a free database.  You could purchase a database query of flights that have been tracked by ADS-B.  Depending on the number of aircraft you need, this will affect the cost.  Flight Aware will quote this kind of job.
The FAA database does not provide this information.  It does provide mode-S code information, but that is whether or not the aircraft is so equipped - nothing for ADS-B (yet).

One way to achieve this (for free) is to join the two following databases:

Set of all aircrafts that operated in US from transportation data
(potentially incomplete) set of all operating aircrafts with ADS-B from adsbexchange


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Why is this 777 zig zagging over the Pacific (and delayed by 2.5 hours in air)?
Why is this 777 (CX 806 on June 6 2017) zig zagging over the Pacific (and delayed by 2.5 hours in air)?

it's a bug in flightstats' visualization. Flightradar shows no such behaviour and says the flight will land approx. 30 min. in advance.

My work site does not allow me to see images here, but looking up that flight in flight aware for June 6 does not show any deviations (and it landed 18 minutes early)...

Sorry my post was wrong -- June 16.

It's not exactly the same question, but the reason is the same, and therefore I suggest to close as duplicate: Why do aircraft on Flight Radar 24 jump around randomly sometimes? Another possibility: Why did the Flightaware groundspeed readout suddenly spike for this flight?

@mins, you mean it isn't just the pilots having fun? disappointed... :)

@FreeMan: Well.... they are having fun in reality, and they also need these tracks to spread their chemicals all over. But you also know... there is this little secret inter-government communication plan to convince people it's a bug in visualization software, using SE. Don't you receive your bonus when explaining the "bug"? Please keep all that under your hat.

It's just a glitch. The most reliable public live flight tracking site is Flightradar24.com (but even it has bugs), and it shows that CX806 has flown in more or less a straight line and will be arriving early.

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What's a typical maximum acceleration of a turbofan-powered airliner taking off?
Whenever I fly on a turbofan-powered airliner, I always notice how much acceleration there is, which seems like more than any car I've ever owned could do.  (That might be an illusion, I realize; acceleration always seems higher to a passenger than to the person pushing the throttle.)  I marvel at the power needed for that much acceleration of such a huge aircraft full of passengers, fuel, and cargo.
What is a typical maximum horizontal acceleration of a turbofan-powered airliner during takeoff?  (Perhaps I should see if there's an app for my mobile phone that can measure and display acceleration before I fly next.)  I'll gladly accept measured real-world values, but if someone wants to calculate an answer, let's assume that we're talking about a Boeing 767-300 on runway 8R at Hartsfield–Jackson Atlanta International Airport, which is 9,999 feet (3,048 m) long.  We can also assume that the pilot runs up the engines to the takeoff throttle setting with the brakes on and then releases the brakes suddenly, just to simplify the calculation.  (I realize that airline pilots typically ramp up the throttle slowly to avoid alarming passengers.)

I think you’d be surprised how low it actually is.  Not sure what kind of cars you have owned, or how hard you drive them, but look up the 0-60 times for sporty cars.  Some may have even more useful 0-100 times published.  Clock the roll next time you fly.  Guesstimate 60mph as best you can.  Guess how fast you are going at 3, 5, & 10 seconds.  I think you will find that airliner takeoff rolls are actually kind of sluggish by comparison.  You probably notice it more because you are thinking about it, and normally drive gently.

Maybe what seems surprising is the "broad power band": the acceleration of a jet starts right away, without a delay for the RPMs to rise, and no pauses for shifts, and it lasts much longer because the top speed of a jet is much higher than a car's.  By the way, I've always had trouble estimating speed down the runway because I'm sitting up much higher than when I'm in a car.  That's one of the reasons I'm asking.  Now that I think about it, I could use an "angle finder" like this one as a crude accelerometer next time.

@rclocher3 There’s an accelerometer in your phone…

there are also apps that will read the accelerometer in your phone and display the info in pretty pictures and graphs.

Calculation
From the wiki page for a B767-300

MTOW = 158.8 ton, OEW = 86.1 ton
Max thrust = 2 * 270 kN

So at MTOW: a = F/m = 540/158.8 = 3.4 m/s$^2$ = 0.35g
At OEW = 540/86.1 = 6.27 m/s$^2$ = 0.64g
When releasing the brakes at full throttle, acceleration transitions from 0 to the above immediately. It is pretty much the maximum acceleration, since at brake release the drag is still zero.
For comparison: my 1999 Holden Commodore is quoted as reaching 100 km/h = after 8.9 seconds.
a = v/t = (100 / 3.6) / 8.9 = 3.1 m/s$^2$

Data from a flight simulator, current passenger aeroplane twin-jet

Compared to road vehicles...

Right, but airliners don't typically take off at maximum thrust.  The pilots calculate how little thrust they can use given the load, the air temperature, and the length of the runway, and then they set the throttle there, in order to maximize the life of the engines.

If they choose to use flex thrust.  Not always.  Depends on the airline's policy, and in the end what the Capt wants to do.  Some pilots don't like to use flex because every takeoff has that short field "end of the runway looming" feeling, even though you are technically within accelerate/stop margins.

Flex or de-rate is almost always used, if available (per performance calculations). Most runways are long enough that you rarely get that "end of the runway looming" feeling. In narrowbodies at least, heavies may be different, haven't flown one.

So the simulated peak acceleration of 0.35 g, which matches the acceleration at MTOW at full throttle, corresponds to a zero-to-sixty-mph time of 7.9 s, or 0 to 100 km/h in 8.2 s.    That's just a little faster than my Mazda Miata (MX-5), which was allegedly capable of zero to sixty in 8.1 s when it was new.  (I know that I'm comparing a peak value to an average value, but I don't have acceleration vs time graphs for a Miata.  I could probably make such a measurement, but I don't want to brutalize my clutch.)

By the way, the 3'4" (3:04) YouTube video in the web page linked in the answer is here.  The spoken language in the video is Turkish, but I got English subtitles when I watched it.  Well worth watching.

Important to include units:

a = F/m

1 Newton = 1 kg m/s$^2$

Mass:  350,000 lbs MTOW ×  1 kg/2.2 lbs = 159,091 kg
    189,750 lbs OEW × 1 kg/2.2 lbs =  86,250 kg

Thrust:  2 × 270 kNewton × 1000 Newtons/kNewton = 540,000 Newtons

a = 540,000 Newtons/159,091 kg = 3.39 m/s$^2$ at MTOW

It would be understandable not to use full throttle if the plane was empty.

One ton is 0.907 metric tons

I've flown a CRJ200 doing touch and goes in a VFR circuit/pattern, with no interior, not even insulation, no paint, and some middling fuel load, not using flex, and the acceleration was... brisk.  And noisy.  Quite a thrill.

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Can small airplanes dump fuel during flight?
Can small airplanes, such as a Cessna 172, dump fuel during flight?
My understanding is that large commercial passenger aircraft often have the capability to dump fuel during flight, as can many military aircraft.  However, is that capability available for small 2 or 4 seat planes?  How big does a plane need to be before a fuel dump capability becomes useful?

A C-172 cannot dump fuel.  As a guess, it's only useful for aircraft where the max landing weight is significantly lower than the max takeoff weight.

The large really needs to be stressed here. Narrow-bodies like A320 or B737 either don't have that capability, or have it as option that most don't have installed. So why should anything smaller have it?

On a C172, the fuel weight is about 150 kg out of 1,200 kg. On a B747-8, the fuel weight is about 200 tons out of 450 tons. The benefit of dumping some fuel is not the same. Also fuel dumping must be done at relatively high altitude, so that fuel is evaporated before touching the ground. See What factors determine where an aircraft can dump fuel?

I saw a question regarding a small plane crash where the poster wondered why the pilot didn't dump all his fuel on approach to avoid a fire.

Fuel dumping becomes useful when the possible fuel load becomes a significant fraction of gross weight, like one third. It has less to do with size and more with fuel fraction and reduced landing weight.

@JanHudec The older Learjets with tip tanks have the ability to dump fuel, yet are significantly smaller than the narrow-bodies that you reference.  The reason though, is that the fuel in the tip tank can slosh around way out there at the end of the wing and cause controllability problems.

The short answer is, no, generally only large aircraft have purpose-built fuel dumping systems. Such a system may be helpful in reducing aircraft gross weight to a safe landing weight in the case of a precautionary landing. The approved landing weight of smaller aircraft, such as the C172, is generally the same as the approved takeoff weight.
The longer answer is, yes, it would be possible to "dump" fuel from an aircraft such as a C172, though not by means of a purpose built mechanism. The "dumping" would be more of a slow to moderately fast drain, depending on the means used.
If I wanted to drain the fuel from one of the wing tanks in a C172 while in flight I could use a pen, screw or other such device to drain fuel from the wing tank sump. A faster dumping/draining method—though requiring considerably more work—would be to unscrew and remove the fuel drain valve, thereby releasing all the fuel in the wing. Obviously, this later method would hamper the possibility of draining only a portion of the fuel from the wing tank.

I cannot think of any reason why I would want to drain the fuel in flight apart from reducing the risk of fire in the event of an emergency landing. And in that case, my prefered method of reducing the risk of a post-landing fire would be to concentrate my efforts on the landing, not on eliminating a fuel source.
Generally, I prefer to have more rather than less fuel in the tanks when I fly.

Even 737s cannot dump fuel in-flight

In addition to your points, systems designed to dump fuel actually spray it to limit / prevent effects on the ground, so it would be in last resort and taking the possible effects into account.

I wouldn't want to try that method either, you are likely to have fuel dripping down your arm and soaking your clothes, and filling the cockpit with nasty fumes. You'd be exposing yourself to tetra ethyl lead which is extremely toxic.

There is one theoretical reason for doing this: if air can't get into the fuel tanks because the wing vent is blocked then the fuel flow will stop at some point. You can let air in by opening a sump, which will restart the fuel flow temporarily; repeat until you can land. But it's a really unlikely scenario: there's an AD requiring vented fuel caps so it should never happen in the first place, and even if it does you would have to actually think about venting and have enough time to try the sump. I guess most pilots would just assume a blocked fuel line and concentrate on landing.

@GdD Not sure about the toxicity as compared to any other lead, but it wouldn't be a problem for anyone running on unleaded, which is common.

172s running unleaded is not common @JonathanWalters. In any case sticking your arm out the window and getting it soaked in toxic, flammable liquid while trying to fly an airplane at the same time isn't a good idea. Doing it while you already have an emergency to deal with is even worse.

@GdD I'll agree with the "not a good idea" part for sure!

Plus if you do crash, you probably don't want aviation fuel all over your sleeve and arm...

On some small aircraft it is possible to dump a small portion of fuel by mishandling the fuel system.
On certain models of Cessna 310, 401, 402, 414, 421 and similar aircraft, the auxiliary tanks feed return fuel back to the main tanks. Normal procedure is to burn down the mains before switching to the auxiliary tanks. If the mains are full the return fuel from the auxiliary tanks will overflow the mains and be vented overboard. Wing locker tanks were pumped directly into the mains and would also cause venting overboard if activated too early.
One time I was asked to take a Cessna 310 to a very short runway to pick up 5 passengers and their luggage. The aircraft had accidentally been fuelled to 1,200 lbs. when only only 600 lbs was needed to complete the trip. The extra 600 lbs. of fuel would have made the aircraft over weight
Shortly after take off I switched to the auxiliary tanks and also activated the wing locker tanks. This resulted in the main tanks becoming over filled and about 400 lbs of fuel was both consumed and also vented overboard during the 1 hour flight.
While this was not an actual example of fuel dumping, it had the same effect, and allowed me to depart a short runway with a full load of passengers and baggage while not being overweight.

So, what you're saying is that it pays to know your plane? Also, wouldn't it have been more practical to off-load some fuel before takeoff instead of dumping it in flight? (Maybe he should ask a new question...)

Removing the fuel from the aircraft would have taken time, and since the fuel is then considered contaminated, it is usually disposed of.

got it, thanks.

As Jonathan mentions these systems are typically not found on light aircraft as there is little need for them but in some cases the design of the plane may allow for it. The Piper Saratoga has has an in cabin fuel drain switch that drains fuel from a center drain at a low point on the fuselage. This is intended for expelling water from the system but can in theory be used to dump fuel.

(source)
Other aircraft may have similar systems as well. The small/older cherokees however do not contain such a system.

of course, having an in-cabin fuel drain actuator can be a source of trouble...

Usually no, although in some cases you may be able to dump fuel by exploiting features of the fuel system. Why not on smaller planes? Fuel jettisoning systems were only ever required for transport-category aircraft.  There is some reasoning behind this link, since many transport category aircraft perform comparatively poorly at MTOW and would take a long time to burn off the difference between maximum take-off and maximum landing weights.  Many GA aircraft can even land at their maximum take-off weights and so don't list a separate landing weight. (See, for example, the C172 POH)
The reason for this discrepancy between small and large aircraft, like many differences between transport category and GA aircraft, is in part due to practicality and in part due simply to regulation.   Fuel jettisoning is required on many transport aircraft but is not required on GA aircraft. The FAA had a regulation that stated (though it's since changed to be performance, not weight-based):

Sec. 25.1001 Fuel jettisoning system.
(a) If the maximum takeoff weight is more than 105 percent of the
  maximum landing weight, there must be a fuel jettisoning system able
  to jettison enough fuel to bring the takeoff weight down to the
  maximum landing weight...
version of 25.1001 effective on 02/01/1965.

There is good reason for the distinction between small GA aircraft and large transport aircraft in certification requirements.  First, many GA aircraft are designed to be able to land and take-off at the same weight.  Other reasons include performance consideration and that fuel flow is not proportional to max fuel, in part because larger aircraft usually have longer ranges. Longer range means more distance to burn off all the fuel, which usually makes it harder to burn off any unwanted fuel.
If you want to be technical, the driving factors here are the performance at MTOW and the practicality of burning off the difference in MTOW (max takeoff weight) and max landing weight, not fuel fraction (fuel available/empty weight) and not necessarily the ratio of MTOW to max landing weight. As counterexamples to the notion that fuel fraction or MTOW/MLW are important, the F7x can hold almost 90% of its empty weight in fuel but can also land at almost 90% of its take-off weight and so has no fuel dumping.  Even a Learjet 60XR, with a MTOW to MLW ratio worse than an A320, can burn off the difference in fuel during a little more than a 300 nm trip. By comparison, by my calculations a 757-200 could make a 2,000 nm trip without burning off the difference between maximum take-off and maximum landing weights, and circling for hours burning off fuel like that tends to horrify passengers. (Source: Plane Purchasing Handbook 2012 and this 757 presentation).
You may also want to read: Why doesn't the 737 have a fuel dump nozzle? and When are aircraft required to dump fuel for emergency landings?

The answer is simple.
If an aircraft's maximum takeoff weight is significantly greater than its maximum landing weight it needs to be able to dump fuel so that it can land quickly.
Small planes, such as C-172, can usually land at maximum takeoff weight. Thus they have no need for fuel dumping capability.There simply is no need for dumping fuel in a C-172 so the aircraft does not have that capability.

The 737 can have a difference between maximum takeoff weight and maximum landing weight of 30,000 lbs, but does not have fuel dumping capability.  While your answer is correct for some cases, perhaps you could make it better at covering  these corner cases where perfomance, not take-off and landing weights, is the driving factor.

This question came from another question regarding a small plane crash where the OP wondered why the pilot didn't dump all his fuel on approach to avoid a fire.

the Mooney M20M has a gross take-off weight of 3368, and a gross landing weight of 3200#.
Although not designed for fuel dumping, it has a operable valve on the floor of the right seat that is used for draining the lowest part of the fuel system, and this valve could be used to dump 168# or 28 gallons of fuel, although it would take a fairly long time.
The better technique is to do a soft-field landing at full gross on a long runway.

Generally, no. However in the Piper J-3 cub.. just fly upside down for a few minutes (you'll then be a glider unless you have an inverted fuel and oil system).  Plenty of fuel will drain out around the metal wire which is your fuel gage. :)

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When calculating takeoff performance why include airport elevation in pressure altitude?
When calculating the pressure altitude for aircraft performance I learned to calculate it the following way:
Pressure Altitude = (29.92" - altimeter setting) * 1000' + elevation
So at an airport at 3000' elevation and an altimeter setting of 29.92" I can expect the aircraft performance to be that for 3000'.
I never questioned that but now I wonder why don't we just care about the altimeter setting? If the altimeter says the station pressure is 29.92" sea level, but I am at 3000', then the airplane still thinks it's at sea level when it comes to performance. Is that wrong?
Same for an airport at 3000' elevation and an altimeter setting of 28.42" I can expect the aircraft performance to be that for 4500'. Again, why not 1500'? The airplane should think that it is, regarding pressure, 1500' above sea level.
I think that would make sense for en-route performance. If the station pressure along the way is 29.92" and I fly above at 4000', my pressure altitude will be 4000' (0*1000 + 4000). If my station pressure however is lower, say 28.92" for instance, then the pressure altitude above at 4000' would be 5000' ([29.92"-28.92"]*1000 + 4000).

If the altimeter says the station pressure is 29.92" sea level, but I am at 3000', then the airplane still thinks it's at sea level when it comes to performance. Is that wrong?

Although the station pressure may be reported as 29.92", that does not mean that a barometer there will show that as the current pressure.  The reported value is the actual pressure, plus an extra amount that is calculated would occur if the recording station were lowered down to sea level.  Instead of actual pressure, you are setting the QNH pressure.
So sitting at a 3000' airport with the altimeter reading 29.92 does not mean the airplane thinks it's at sea level.  Certainly the altimeter doesn't think this, because it's reading 3000'.
You could use the actual pressure outside as an altitude-independent performance gauge, but you don't normally have that value (and ATC isn't going to radio it to you).

A manifold pressure gauge will read atmospheric pressure when the engine is turned off.  Thats as close as you'll get in the airplane though.

Another way to complement ymb's answer - if you set the altimeter to zero as in the picture on the left, you will get the static pressure at the field - your altimeter will then read AGL and not MSL - and we fly MSL - one way to think of why we put the sea level pressure in the Kollsmann window is that we are making that air inside the aneroid wafer equal to the pressure at sea level - the altimeter should read within 75 ft of field elevation - as the airplane rises, the static pressure decreases, causing the wafers to expand - so the starting point (field elevation) is then adding feet as the aircraft rises.

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How to identify a NDB on a garmin 430W or GTN650?
I have a Garmin 430W and a GTN 650 interfaced with a G500H in my aircraft. I can program either to go direct to an NDB. The question comes as to whether or not I can identify the navaid. Reading through the manuals I cannot determine how I would do that? Thanks in advance!

I don't know anything about the GTN 650 but the 430W has no ability to receive NDB signals, so at least you won't be able to identify the navaid using that equipment.

So when I put in “direct to “ to an NDB that is essentially a gps fix?

Yes it's just a GPS waypoint.

On the GTN 650, you can go to the waypoints page and enter the name of the NDB. It will now show distance and bearing to the NDB from your current position. See pg.180 in the GTN 650 manual. Hope this helps!

Question already says that programming the course is possible.  You haven't answered the question, which is how to confirm reaching the navaid.

I did answer it. It shows distance and bearing. When you reach the navaid the distance will be 0NM and the bearing will swing to the opposite direction you have been flying on similar to a VOR cdi but the whole needle will swing for NDB. It will do this both on the GTN650 and on your PFD (blue needles) if they are connected. In your question, you are referring to it more on a GPS level programming into your flight plan/direct but the way I am saying shows it more like a real NDB (generated by your GPS). Pg. 180 explains it well.

It won't identify the navaid, just the place where it believes the navaid to be.  I think that Wes is looking for physical confirmation, but perhaps I'm mistaken about that?  The question could certainly benefit from more clarity.

Yes maybe some more clarification but if your aircraft cannot receive NDB signals you cannot directly connect to the NDB (obviously). The way I (and the manual) mention it is GPS in lieu of NDB. This is by using the NDB in the GPS database (if it is available which most likely is). This is similar to using GPS in lieu of DME on approaches. This is a legal way to identify the Navaid, we do this all the time in approaches using GPS in lieu.

Taylor B thank you for that! I believe that is why I cannot identify it is because it is essentially an GPS waypoint in lieu of an NDB. Thank you for that clarification!

Of course! Glad it helped!


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How a power assurance check works?
In a Bell 407 the power assurance check takes into account torque, pressure altitude and MGT (measured gas temperature aka TOT). The flight manual states that if MGT when tested is less than or equal to the charted MGT, engine performance equals or exceeds minimum specifications. So my question is how does MGT factor into the health of an engine. Torque gauge meters off oil pressure, MGT is a result of compressor performance or bleed air systems. So an engine could be a plus engine with let’s say bad compressor vanes, or a bad bleed air valve etc. So my initial question is what is the correlation between MGT and engine health.

Think of it this way.  When an engine is temperature limited (as opposed to being torque limited) it means that the power control can be advanced (thus making more power) until the max specified engine temperature  is reached.  if one engine produces a particular amount of power while at a lower temperature than a second engine when producing the same amount of power then that first engine has more “room” to further increase power than the second one does.  Thus, the first one performs better.   In other words, the cooler the engine is at a particular power output level the better the engine.
The manufacturer puts out engine performance tables that specify what the nominal engine performance should be. If the engine runs cooler than that then it is better than spec.

Thank you very much Jim I greatly appreciate your answer!

@Wes Showing appreciation is done by voting and accepting questions. :-)

The difference between the max ITT/TOT/EGT and the measured value is "ITT/TOT/EGT margin". The margin is highest when the engine is new, and declines as the engine wears out.  As compressor and turbine blades erode, efficiency declines and more fuel is burned to make the same torque or N1 or EPR, so temps go up.  When the margin gets to some value close to zero, that is, the measured temperature is close to or at the engines limit, the engine is worn out.  The power assurance run is mostly to monitor and track the margin over time.

Thank you John that makes perfect sense! Easy enough to comprehend I just couldn’t put it together on my own! I appreciate you sharing your knowledge with me!


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If the updated MCAS software needs two AOA sensors, doesn't that introduce a new single point of failure?
Regarding the 737 MAX story, the New York Times writes:

"Boeing’s software update would require the system to rely on two
  sensors, rather than just one, and would not be triggered if the
  sensors disagreed by a certain amount, according to the three people.
  Given that the 737 Max has had both sensors already, many pilots and
  safety officials have questioned why the system was designed to rely
  on a single sensor, creating, in effect, one point of failure [emphasis mine]"

Now I understand that this avoids a False Positive, when one erroneous sensor triggers the MCAS.
But, considering the opposite situation, doesn't this update introduce a new single point of failure, a False Negative, when a stall should be counteracted with MCAS but it isn't, because only one sensor detects it?
(Or if not, what am I missing here? Is it that a faulty sensor fails in a certain way and will not read normal AOA erroneously?)

Noting that the details of the MCAS update are yet to be publicly confirmed by Boeing - no I don't believe you are missing anything.
MCAS was meant to be a system that only kicked in when the pilots were letting the situation get out of hand. It was to aid in stall prevention, but does not do anything the pilots can't (as long as their situational awareness would allow). In a million flights MCAS would not be used once unless there were other serious difficulties at play.
In such a system it is much better to have a false negative than a false positive. A false negative means that the aircraft doesn't change anything, and continues to follow the pilot commands. A false positive means... well, it looks like there are 2 crashes that demonstrate what happens.

Another way to look at things is to say that inaction by MCAS can never represent a single-point of failure, since the only time MCAS would need to do anything would be after there have already been two points of failure (typically one involving the captain and/or the controls operated thereby, and the other involving the first officer and and/or the controls operated thereby).

Every automated system has a possibility of a false positive and a possibility of a false negative.  In the system design you have to consider
( Probability of a false positive * consequence of false positive ) versus (probability of a false negative * consequence of false negative).
A team of engineers at Boeing certainly looked at the tradeoff above in the initial design.  The probability of AoA sensor failure was most likely based on failure rates from historical aircraft such as original 737.  The consequence of each failure was presumably a little harder to estimate, because no such MCAS system existed on previous aircraft, but they somehow they came up with an estimate of what would happen in each case.  Based on that, they believed they had the right tradeoff.
Now, new information has come to light.  Specifically, "consequence of false positive" is an absolutely unacceptable situation (two fatal crashes).  Therefore the system needs to be redesigned.  A increased probability of false negative may be acceptable, if it can significantly reduce the probability of false positive. Both errors are still possible, and both consequences still exist, but the tradeoff is shifted to favor one versus the other.

A positive should not activate an uncommanded downtrim to the extent that the aircraft is unflyable, although whether or not it automatically activates could be discussed.  Lack of override training was fatal.  The MCAS itself wasn't, but could be improved.

@RobertDiGiovanni MCAS failure looks exactly like runaway trim, which B737 crews have supposedly been trained to handle for decades. Indeed, many MAX pilots in the US have reported handling it fine, so Boeing's decision seemed sound. However, they didn't account for foreign pilots not being as well trained nor how often the system would fail.

So more needs to be done.  Good idea, bad execution.  Yes, better training.  And more input from the pilots.  It could be better.  If the 737 is at the end of its design life, this (and the placement of the oversized engines) may have been one step too far, but continued research and development of MCAS may help save more lives in the future.

@StephenS MCAS failure does not look like runaway trim. Runaway trim would be a constant movement in one direction or the other. The MCAS did not do this. It would trim nose down, but the pilot trim input would stop it and trim it back where they wanted it. Then there would be a few seconds before it activated again. Without knowledge of MCAS that behavior would be very confusing to the pilot.

I understand the balance between FP / FN, my point was the concept of a single point of failure. Not every automated system has a single point of failure. Especially aircraft.

@DanielSparing, A single point of failure is not necessarily a dealbreaker in design if the overall system remains acceptable, and conversely a triply redundant system may not be any safer than a single one if not designed properly (look up Sioux City crash). Again, all comes back to weighing probabilities times consequences.  Reducing single points of failure is certainly good, but sometimes there is no better alternative

The new system will not be a single point of failure.
Normally, the AOA sensors should not disagree.  But then again, normally pilots should not be flying the aircraft near stall margins.
However, if the sensors do disagree --  it will tell the pilots with a cockpit indication: effectively "MCAS will not rescue you today, watch your trim".  It should also automatically log a report of the failure to the maintenance staff.  This then becomes a maintenance item that must be fixed soon.
You're right that either sensor failing will cause this, and you're right, that is a single point of failure of the MCAS system; but this would still require an unbroken string of pilot mistakes to cause a crash, and that string of mistakes isn't happening today on the thousands of 737 classic and NGs without any MCAS at all.

Having two of the same type of sensor may not improve things as icing conditions could easily cause disagreement just when it was needed the most.  A second system, such comparison of airspeed, pitch to the horizon, power setting, and vertical velocity (In addition to what the pilots are doing) may be much more useful.
Grossly changing the horizontal stabilizer pitch in an uncommanded manner only worsens the situation when the pilot needs to be in control.  Breaking a stall is done by releasing the elevator.  A properly designed air craft will almost immediately unstall, especially if it is caught early.  Strict adherence to aft CG limits greatly improves safety as well.
A more pilot friendly MCAS may work as follows.  Design the elevator such that, in conjunction with the horizontal stabilizer, it does not have enough pitch authority to stall the plane under normal flying conditions.  An aircraft of this type, with a properly set CG, at full aft elevator, will lose airspeed, start to sink, and "mush" forward with the nose dropping.
Have amber and red stall warning lights.
If a stall warning occurs (real or not), pilot and computer check second system data.
If stall is real, pilot activates MCAS. (toggle switch)
The MCAS would ONLY increase the elevator throw rate and travel. Much like dual rates in
R/C planes, this would hugely increase pitch authority, but would always be under the control
of the pilot.  Once stable flight is restored, the pilot turns off the MCAS.
Best luck to Boeing getting this fixed.

The compounding factor is that the B737 trim moves the entire stabilizer, so even full elevator deflection can't undo incorrect trim. One solution would be to auto reverse trim when fully deflected the other way.

That scares me.  With a dual rate elevator there would be no loss of control (although high rate would have to be handled very delicately).  Stabilizer trim would be there as a backup.  Traditional hierarchy is Hstab stronger than El, El is stronger than Trim.  I imagine coarse trim for Hstab, and fine trim for trim tab (yet another potential backup).

@StephenS Under normal circumstances there is enough elevator authority in the 737 to counter full nose down trim. The situation here is slightly more complex in that it appears the Lion Air pilots increased speed due to the incorrect stall warning. The increased speed may have limited elevator authority due to blowdown.

So, do they need to make the Hstab stronger? Another tidbit, when McDonnell Douglas merged, the MD90 and 737 filled similar roles.  The MD80/90/95 lived on as the 717 before production ended.  Their engines were rear mounted.  A development was a flaps system mounted near the engines to assist pitch down in stalls.  Could this be a precursor to a vectored thrust solution?

This does not seem to answer the actual question but seems like more of a general essay on how you feel the MCAS system should be designed.

Perhaps, but there was concern that 2 of the same sensor would still be a single point and would not significantly improve the situation.  That is why I suggested a secondary computer augmented assistance to review AOA and stall condition by using airspeed, pitch to the horizon, vertical velocity, and power setting (as a pilot does).  For example:  250 knots, 0 vertical climb/descent, normal cruise power, 5 degrees above horizon, am I stalled?  Even if AOA says yes, pilot knows better.  Felt a need to say something. Hope they reconsider the 717.


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How to approach flying a new aircraft?
What's a few of the first things you look at before flying an aircraft you're unfamiliar with. Do you look at emergency procedures, V speeds, or weight and balance. I'm just curious on what other experienced pilots have done in the past when approaching a new aircraft. Do you have your own system on what you look at first? I'm sure some guys and gals have made mistakes and have learned from it, so I'm just wondering if there was something you do particularly when in a new frame.

I'm voting to close this as it's both broad and opinion based @DavidHopkins. It's a good question to be asking, just not a fit for this site.

The first step to flying an unfamiliar aircraft should be to get an instructor who is familiar and take some lessons.

very carefully and circumspect. Don't do as John Denver

How to approach flying a new aircraft?

Find an instructor. Period.
Your instructor will, based on your past experience, guide you through the transition. For example are you going from Cessna 152 to 172? Or Cherokee to Cessna? Do you have experience with manual trim? Glass cockpit experience? These are just some examples. The process is so dynamic & variable that one cannot give a concrete answer here.
Say you have a PPL, and you want to exercise your privileges on this new aircraft. Your task is simple: demonstrate that you can perform what a PPL is supposed to perform if you were to do the checkride in that aircraft. You can expect to go through the entire PPL syllabus, only at a much faster pace because (hopefully) you understand every concept, it is just the matter of applying them to a new aircraft. Your instructor will not have to explain theories to you; it will be a lot more flying and a lot less ground school.

First off, unless you’re a seasoned test pilot under the employ of a large aircraft OEM to do experimental flight test - SEEK INSTRUCTION FROM AN AUTHORIZED INSTRUCTOR PILOT QUALIFIED ON THE AIRCRAFT YOU WISH TO FLY.  It’s not only a sound piece of advice but in many cases, it’s also the law.
The basic process of transitioning from one aircraft type to another involves 1) a thorough understanding of the aircraft, it’s specifications, performance and limitations. 2) a thorough understanding of the onboard systems, how they interact, and consequences of failures of one system upon the others and to the safety of flight. 3) The handling characteristics of such an aircraft during flight operations.  4) Dealing with emergency situations.  These are very general guidelines and the concrete implementation of them will depend on the specific aircraft.
As I said before the transition process should be done under the guidance of a qualified flight instructor following manufacturer approved literature for that specific aircraft eg AFMs, checklists, etc..  You will also have to ensure that the pilot certificate and ratings you hold allow you PIC privelages in the type of airplane you want to fly.  Small aircraft will typically  only require you to hold at least a private pilot certificate with ratings for the appropriate category and class of aircraft you wish to transfer into.  Larger aircraft such as those over 12,500 lbs or turbojet powered will require a type rating, a specific training regiment of knowledge and and practical work culminating in a check flight and the issuance of a rating to fly this specific aircraft type.
Even when approaching changing between light airplanes, there are often performance differences between the two which can have disasterous - even lethal - consequences when not approached by following this philosophy, as these examples show.

A graphic example of why you should always seek out a proper transition program from a qualified flight instructor with experience on the make and model.  Airplanes are not cars; there are often significant differences in handling qualities between two types.

Why a difference in airframe with performance and takeoff configurations recommended can have disastrous consequence if not fully understood.

Transition to a different aircraft with different handling qualities and avionics should not be taken lightly without a proper transition period.

Again, differences here in performance and handling qualities between two airplanes require time to get used to.

Yeah I was looking for more general items. I'm not talking about from jet to jet. I'm talking about like a cessna 172 to a Piper cherokee. Airplanes you're rated to fly in, just a different make and model.

Follow the advice given, then.  It applies to light GA aircraft as well.

@DavidHopkins it's no different, just the process will go quicker (maybe take a few hours rather than days).


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Can light aircrafts with no retractable landing gear make an emergency water landing?
In the suburbs of Lisbon today two people were killed because a pilot of a light aircraft (I imagine without retractable landing gear) landed on one beach full of people (it's Summer and the beaches are crowded).
The question immediately arose on the public opinion: can those light aircrafts, with wheels, land safely on water? This aircraft seems to be a Cessna 152.
The other post "What are the risks in landing on a beach vs. shallow waters?" does not solve this question.
Can light aircrafts with no retractable landing gear make an emergency water landing?

This answer to the duplicate question clearly answers this one; the first answer also contains plenty of relevant information. Voting not to re-open.

@DavidRicherby the questions are clearly different.


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Why is a plane escorted while in technical difficulties?
In a recent incident, where the pilots of an Embraer 190 lost control and started flying erraticly in Portugal, two fighter jets started escorting the troubled plane after the emergency was declared.
Why did they escort it if it wasn't a security/military threat? What could they have done or how did they help?

I'm not sure this is the answer, but they are trained in close-quarter maneuvering and can provide real-time assistance to the pilots in case they had to put down somewhere other than an airport.

Certainly a situation where, if the plane were out of control and headed for a densely populated area, a decision would have to be made, especially post 9/11 incident.  If they were able to communicate with the stricken aircraft, help as far as airspeed, direction and visual inspection of damage would be helpful.

Related, although the reason is a bit different

It's not a common thing, as aircraft have technical problems every day without a jet escort. Generally only extraordinary circumstances would merit it.
In this case, the pilots were struggling with an uncontrollable aircraft as well as difficulty navigating - there was lots of bad weather about, and they repeatedly asked for directions to the ocean to ditch, indicating that they didn't know where west was. The jets were scrambled and were available to help the pilots navigate through the bad weather and towards a safe airfield (I don't know if the navigation systems were malfunctioning or if the crew were just understandably overloaded). Fortunately the pilots were able to get the aircraft under some control, I think before the jets even did anything (but more information might come out soon).
Fighter jets can also help check for damage - being better trained and more maneuverable that a civilian craft allows a closer inspection. An underrated benefit is by just being up there with them, panicked pilots can calm down and focus on the task at hand.
Finally there is the horrible option of shooting the jet down if it has been hijacked. This would absolutely be a last resort though, and it's possible that these fighters weren't even carrying weapons.

and the jets can help clear the airspace around the aircraft in trouble by blocking other aircraft from getting too close that may have failed to be appraised of the situation by ATC (yes, you VFR pilots out there that failed to listen in on the appropriate ATC frequencies, I'm talking about you).

Pure speculation (on my part), but I think you're right on the money with the comfort of knowing "sombody's out there". Even though you know there's nothing they can actually do for you, having another plane/person there is often very settling.


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How much bigger could you realistically make the Antonov 225?
The Antonov 225 is the largest cargo plane in the world. But how much bigger could you REALISTICALLY make it if money wasn't a problem? Could it be say twice as wide? Carry twice the weight?
What materials and physics constraints are engineers up against? Is there a limit on today's materials to make a larger cargo hold or wings that could stand the bending moments of more engines and fuel in the wings? Would runways not be able to stand any extra weight than the 225 can already carry?

You should check out the Scaled Composites Stratolaunch, which is currently the largest plane by wingspan in the world.

Having said that, engineers solve all the challenges they run in to when they designed these behemoths, but no more. I think your question is impossible to answer until someone to design such a plane.

"Realistic" and "if money wasn't a problem" are inconsistent assumptions. What is realistic is driven as much by economics as by the theoretical (but only achievable at extreme expense) limits of engineering. How big could it be if the whole thing was built with titanium & powered by hydrazine? Pretty huge, I bet. But is a billion-dollar-a-copy Antonov realistic? Pretty doubtful.

@RalphJ I don't necessarily see the contradiction. You can ask what is in the realm of possibility given the materials limits, but forgoing the financial ones. Sure, this makes it a "theoretical" limit, but that's easily fixable semantics. Plenty of scientific studies look at what's "theoretically" achievable.

oh, hey, a related question with 79 upvotes! I wonder what's happening to the people here that need to downvote so quickly everything that they don't know how to answer.

@Federico Biggest aircraft theoretically possible with unlimited budget is an interesting question. Biggest aircraft economically viable is also interesting. But trying to ask both at once with "REALISTICALLY" specified in capital letters misses how different the 2 questions, and their  answers, are.  What could be done limited only by engineering constraints but with unlimited budget, is in no way shape or form even slightly "realistic."  Is "Saturn V" an answer to "biggest rocket I could realistically build in my back yard, if I had no budget constraints and unlimited space?"

@RalphJ you've just repeated your previous point. I could repeat my comment. I will just add this: consider that not everyone has English as their first language, and that what you understand reading does not necessarily equates to what was meant by the writer. A bit of flexibility when interpreting could help. It is clear what the OP meant by "realistically", no need to be pedantic. You can either ask for clarification, or edit with the correct terminology. Who do you help by simply criticising?


text:
Is fuel moved around during flight on a passenger or cargo plane? If so why?
Is fuel moved around during flight on a passenger or cargo plane? If so why? Is balance an issue?

Related: https://aviation.stackexchange.com/questions/42382/has-anyone-researched-or-built-an-aircraft-that-controls-pitch-and-roll-by-pumpi

Thank you for all your answers - very interesting. Concorde is really unique! What about other unique planes like the huge Antonov or the a380? Does size make a difference?

Don't forget that fuel is always moving to the engines - where it is converted to thrust and loss of mass - when they are operating.

maintaining a proper center of gravity is important so you don't waste fuel using airflow to maintain your velocity.   If you get tail heavy, for example, then you must use your elevators to maintain your pitch, which creates drag which costs fuel.

At the very least it has to be moved from the fuel tanks to the engines, without which you're flying in either a very bad glider or a brick with aeronautical pretensions. :-)

Maybe
This is dependent on the air frame and varies from plane to plane and not all planes have capability of moving fuel although most large planes do. The Concorde moved fuel all over the place from its 13 tanks to cool the nose cone as well as trim the aircraft. It was perhaps one of the most complex implementations of such a system and kept the flight engineer quite busy.

(source)
Here is a layout of how it was used to control trim.

(source)
For any of this to be possible the aircraft must be equipped with cross-feed abilities. On larger planes this is fairly common, in case of a single side leak the cross feed valve is usually kept shut but since engines can have slight variances in fuel burn the pilots may need to cross feed every so often to balance the plane out. There is a nice overview on 737 fuel systems here.

(source)
In small, often single engine aircraft fuel management is generally left up to the pilot. There are often multiple tanks which can be individually selected and must be rotated in flight every so often (30 min usually). Since some planes are prevented from drawing from "both" tanks, the switch has a lot to do with not running a tank dry in flight.
This article overviews weight and balance fairly well. Often times quantity of fuel is more important than location. Most modern airliners store the fuel fairly close to the CG (in the wings or belly for that matter). The Concorde's unique delta wing design led to a fairly different use case than your run of the mill jet.
Some of the balance issue is solved within the tank using baffles which keep the fuel from sloshing around too much but allow it to flow freely around a fairly large tank when need be.
You can find the full text of the FAA's book on fuel systems here.
Here is a neat video of how to actually cross feed fuel.

Yes fuel is moved around, and balance is indeed an issue. The A330 has trim tanks in the horizontal stabiliser, and so does the A380. Fuel can be stored there to balance the aircraft - without trim tanks the stabiliser would be set to produce lift to balance the plane, and this causes extra drag.
In the A380 fuel transfer occurs automatically, unless a fault occurs of course in which case the flight crew will initiate the fuel transfer. From the A380 FCOM:

The fuel system stores fuel, monitors the quantity and temperature of fuel in each tank, and controls fuel transfers to:

Supply fuel to the engines and to the Auxiliary Power Unit (APU)
Maintain the Center of Gravity (CG) within limits
Reduce structural loads
Control refueling and defueling
Enable fuel jettison, if necessary.

There are 11 tanks that store fuel. The feeder tanks:

Contain a collector cell of about 1,300 kg that is kept full in order to be able to maintain engine fuel feed at negative g.
Are positioned such that a rotor burst does not affect them.

Fuel transfer takes place from inner tanks first, then mid, then trim, and outer wing last to maintain wing bending relief.

In 747-100/200 aircraft, takeoff was accomplished tank-to-engine (meaning each engine was fed from it's corresponding main tank). After takeoff the fuel burn schedule called for center tank fuel to be used after takeoff until exhausted. This was done by turning on jettison/override pumps in the center tanks that put fuel from the center tank into the common fuel system ducting used for refueling and for jettisoning at a higher pressure than the tank-to-engine pumps.
The center tank was that tank that was typically used to bring the CG of a tail heavy freighter to within limits, which is why the zero fuel weight CG of a 747 was checked and why fuel used to balance the zfw was considered ballast and not available to burn.
Three-engine ferry flights were allowed, as were continued flights after a single engine failed. In these cases the flight engineer would periodically transfer fuel out of the failed engine's tank to the other side.
The TWA 800 accident in the late 1990s complicated center tank fuel usage, but that's another topic.

Does that mean you could get a fuel emergency even with plenty of fuel in the center tank, because using that would put the CG too far aft?

@Sanchises In the 747, common sense would dictate that you would use the fuel and accept the CG moving beyond the aft limit. Whether or not a CG just aft of the limit would cause real difficulty would depend on flight conditions, and in the great majority of cases there would be no problem. If you were concerned, you could carry extra airspeed on approach and into the landing flare. In the operating world I had up until retirement in 1999, if you burnt a little ballast fuel, nobody would ever be the wiser. The flight engineer would ensure that the paperwork was copacetic.

@Terry thanks for teaching me copacetic

In addition to Andrew Morton's comment regarding moving from tanks to engine. It is sometimes also possible to move it outside of the plane, i.e. Fuel Dumping.
This is used to reduce weight of the aircraft to be able to land without overweight. And also for "Dump-and-burn" maneuver (dumping and igniting with the afterburner) to create a big flame for visual effect during air-shows.

Dump-n-burn requires an afterburner to ignite, no? Not generally available on the airliners in question afaik…

Yes, and dumping is not always available either

And fuel dumping can be useful even in absence of dump-and-burn capability.

Yes, fuel is moved around during flight in an airplane.
As the plane flies and burns fuel, the fuel level in the tanks will lower.
If you consider the elevation of fuel within the tank as a Y-axis value, then gradually as flight progresses that value of Y will decrease. So yes. Fuel moves around during flight.
If the fuel tank is symmetrically distributed about the aircraft in the X-axis then balance would not be an issue as it is moving in value only in the Y axis

In an aircraft with 4 engines and 11 fuel tanks, not every fuel tank is coonnected to every engine.. Balance is an issue if fuel burned is not located at the CoG..

Ask what time it is, you'll get told how to build a watch.  Yes, fuel is moved around during flight.  As fuel is used from a tank on one side, that side becomes lighter, and that wing tends to want to rise.  By switching to a tank on the otheside, the drain of fuel from the other side lightens that side, and that wing slowly rises, balancing the plane and making it easier to main level flight.  This happens automatically on large planes, but it can also be done on some of the better smaller planes.  Helps the plane maintain a level attitude during the flight so the pilot (or autopilot) doesn't have to fight to keep the plane level.

authors: Bob Jarvis - Слава Україні


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Wind shear questions
I had a discussion and I'm curious on an experienced pilots knowledge.  When taking off, let's say from telluride. You have a tailwind, but when you get off the ground you experience and windshear that turns into a headwind dramatically. What are the hurdles you'll face? At telluride it says to only land and take off on certain runways, so I know this could possibly happen. Also would the lift generated from the sudden increase in headwind cause the AOA to start increasing without you pulling back on the yoke???

A headwind gust for a given elevator trim condition will result in an increase in airspeed,
which will pitch the nose up.  The aircraft will also "balloon" upwards from increased lift.
Vertical rise will contribute to lower relative wind, but will also put additional downward pressure on tail, again raising the nose.  The effect will be temporary as AIRSPEED will rebalance to the air mass.
As always, the trim will only allow the AOA to increase a to a point before the tail goes from lift negative (downforce) to lift positive (upforce), preventing further increase in AOA.
An excessively forward CG with greater trim will make these effects more noticable.
The real danger under these conditions would be turbulence from the wind shear and
mountainous terrain.  In light recreational aircraft, one may wish to be very cautious about
air travel until weather improves.  In the mountains, wind speed and direction can be extremely localized and variable, and should be carefully monitored.


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How many lives would be saved if the airlines turned all the seats around?
The psychology and history of forward-facing seats has been answered well already here. I ask instead about how bad it is that seats are forward-facing.
For context, I was looking through Wikipedia's list of airline flights that required gliding, and a surprising fraction of the 30 incidents listed were fatal to some, but not all, of those aboard (47%). Averaged across the 30 incidents we get that 25.1% of those aboard died (noting that this average is pretty unscientific, and is subject to sampling and coverage biases; it's still interesting). Even among just incidents with fatalities on average less than half of those aboard died! I generated a scatter plot of the incidents:

This data surprised me a lot! My layman's intuition says that emergency landings should be a lot more bimodal, with basically all landings either being gorgeous US Airways 1549 style happy endings, or blazing fireballs with no survivors, with some very rare exceptions. Based on this intuition, I always figured it was reasonable that passenger seats face forwards; I thought "rear-facing seats wouldn't help that much anyway, edge-cases where it would have made the difference will be the rare exception to the rule".
But is this really true? It seems that maybe many deaths on commercial flights are incidents in this intermediate region, although I don't have the expertise to evaluate if rear-facing seats would help with how people typically die in commercial airline accidents.
Question: As a really rough ballpark, how much safer would commercial airline flights be (averaged across all commercial flights globally) if the seats were rear-facing?

The problem with rear facing seats is it's uncomfortable on takeoff, where the maximum G's are put on the passengers, and the passengers are tilted downward as the plane rotates. Anyone who has ever flown the Southwest 737's with some rear facing seats has experienced that. Not the end of the world, but not comfortable. In any case, the fatalities in crash landings are usually from the fire that follows, not blunt force trauma. How you sit won't make you any less vulnerable to fire.

"the fatalities in crash landings are usually from the fire that follows, not blunt force trauma" Very interesting, this is precisely the sort of information I was looking for. Do you have any citations on causes of death in airline accidents that could let us upper-bound how much safer rear-facing seats could be? For example, if only 50% of passengers die of blunt trauma then it's pretty implausible that this would more than double safety (although, I suppose technically people could be so shaken up from a front-facing crash that they can't escape a fire, or something confounding like that).

It will cause fatalities because people will drive instead.

Excellently phrased first question. Welcome to Aviation SE!

@mins Ah, yes, those intoxicated passengers are terrible in crash landings. They keep charging the galley and drinking all of the alcohol as soon as the plane hits the ground. Very hard to get them to evacuate while they're drunk. ;) On a positive note, after they drink the alcohol, there's less fuel for the fire.

@tj1000 hypothetically it could, if it made getting out harder due to (otherwise) non-fatal injuries or unconsciousness.  Even people being more disorientated could slow evacuation (e.g. if told to use the rear escape slide, and they have to move forwards in their body's reference frame to do so, some poeple are goign to start moving the wrong way)

Also @tj1000 I travel by train a lot, and there the deceleration is more noticeable (and more unexpected) than the acceleration.  In that case I find a rear-facing seatmore comfortable, which appears to support your point.  Some people dislike rear-facing seats because of travel-sickness

Turning all the seats round won't save lives. Leave the pilots facing forward.

@BrianDrummond haha - That reminds me of one of the controllers that used to work JFK Ground. Pilot, after getting clearance to push back: "Which way would you like us to face?" Ground: "Towards the front. It makes the passengers nervous if you face the back."

@reirab Why would Ground care about nervous passengers on an airplane? :)

Don't underestimate the number of people who get motion sickness.  Multiple people barfing on flights will quickly erode the popularity of rear-facing seats.

Would data from high speed trains be of any use? They routinely have both forward- and rearward-facing seats.

As a result of this question I'm now wondering why planes don't have three-point seat belts. In cars this results in a significant reduction of fatalities (see: http://www.roadsafetyobservatory.com/HowEffective/vehicles/seat-belts) Studies also indicate three point belts are especially effective in frontal chrashes, e.g. extreme deceleration. One study (Maghsoodloo et al 1989) even claims there is no major benefit of lap belts in preventing fatal injuries in frontal crashes.

Probably not as many as would be saved if the airlines turned themselves around. Let's get on the ball, here! Our lives are in these people's hands!!

I hate to say this, but the place to spend the money is on accident prevention, not on the marginal reduction in injury based on a seat mod.

The evidence is not the most recent one, but

According to an article in the December 1952 edition of Naval Aviation News, “Passengers in Navy transport planes have ten-fold better chances of coming out of crashes alive, thanks to backward-facing seats which are being installed in all new planes […]". The unsigned article cites two Royal Air Force accidents involving a four-engine Hastings and a two-engine Valetta. Both had rear-facing seats that were credited with minimizing injuries to passengers.

Unfortunately, the link to the 1952 edition ends in a 404. Thanks to the detective work by Gerald Schneider we now have a working link. The article is on page 27. Another source cites a 1957 study, without a traceable source:

An Air Force study in 1957 reviewed a series of crashes and concluded
  that injuries were seven times greater among passengers facing
  forward.

That a backward-facing seat is safer is indisputable, but quantitative data is very hard to find. Given the much higher wing loadings and landing speeds of modern airliners, possibly those ratios cannot be translated to modern aviation. While in the days of propeller-driven transports the difference between the forward- and backward-facing position might indeed have meant that the latter provided a significant advantage in survivability, the deceleration level in a modern airliner crash might make both equally unsurvivable. In addition, the data from mostly young, fit and healthy subjects involved in crashes of military transports might not translate easily to todays untrained, middle-aged and overweight passengers.
Circumstantial evidence cited in this 2016 study, however, indicates that even for modern airliners seating position is important. For an A340 runway overrun accident in Toronto in August 2005 it cites the Canadian TSB as saying:

‘One of the cabin crew, seated in the same general area as the crew
  and passengers who incurred serious impact injuries, was not injured.
  This cabin crew’s seat was aft-facing; the other seats were
  forward-facing.’

Another more recent paper gives more modest advantages: This 1996 study by the European Transport Safety Council lists them in a table and awards rearward-facing seats with only a 5% advantage overall or 19% if based only on impact fatalities:

Commenting on the table, the numbers are based on the 1995 Cherry study (CHERRY, R.G.W. and Associates Ltd (1995) Analysis of factors influencing the survivability of passengers in aircraft accidents Vol. I-III. EU DG VII contract) and

These particular figures are not quoted in the study, but have been
  calculated from the data provided in the study report.

Very informative, thanks! I'm trying to read the report to figure out what the other 3/4ths of the "overall fatalities" are such that "impact fatalities" are only approximately 1/4th of them. It looks like a bunch of it is fire-related, but what I'm seeing doesn't quite add up... can you decipher it?

@JupyterUser: I've added another report (from 2016 this time) with plenty of references, most of which with an active link. However, for seating position it seems to copy over the conclusion from the 1996 study. The 1996 study says that "Just over 50 per cent of all fatalities in survivable or technically survivable accidents are due to impact", so it creates a somewhat arbitrary distinction between survivable and non-survivable / non-avoidable accidents. The 5% apply to all accidents while the 19% apply to all avoidable fatalities. Makes quite a difference in the statistical base!

The .pdf of the 1952 edition of Naval Aviation News is available on archive.org.

Interesting that floor strength is an even bigger factor according to this. I guess that this is because the seats don't shear off and go flying around?

Even if it's only a difference between a passenger getting a broken leg or not I guess it's a lot easier to evacuate a plane (and get away from fire and smoke) when the passengers are not impaired by wounds

I myself for some time thought about the idea of rear facing seats. A problem I see is that in an airplane crash oftentimes the contents of the overhead compartment and fragments of the overhead installation (lid of the oxygen masks, plastic around the reading lights,...) are sent flying through the cabin.
Usually, this debris would hit the seats, or the passengers backs while in the brace position. In rear facing seats, you probably would not go into brace position as to exploit the cushion effect of the backrest. This leads all flying debris to be targeted at your face and upper torso. And this made me question the benefits of rear facing seats.
http://www.dailymotion.com/video/x1q2nag
In the video at ~1:13:00, it can be seen that during a crash, overhead installations do not necessarily stay where they belong. At ~1:17:30 we hear that flying debris and upright position do not really mix. (In this case the debris is moving rearward[?])
Also the table in Peter Kämpf's answer supports the claim that breaking/opening overhead stowage is indeed a source of danger.
In the 1950s studies Peter Kämpf cited, the beneficial effects of rear-facing are about military aircraft - and I am not sure that they have overhead stowage.

Yes flying overhead debris is a danger. Personally, I'd rather face that danger with an intact body, than be protected from it by the back rest with a broken hip from the deceleration. We're supposed to lean forward in those tiny economy seats anyway. By the way, you should be able to comment now.

The danger of flying debris can be somewhat overcome by introducing an alternate brace position where passengers cover their faces with forearms, if they are rear facing, don't you think?

In a crash, the debris is moving forward relative to the cabin, not aft. Or, more precisely, the cabin is slowing down its forward motion more quickly than the contents of the cabin. So, yeah, flying stuff will hit you in the face if you're facing aft.

This would most likely be taken into consideration when designing the airframe if the seats are facing backwards :)

The study recommends to apply the same construction standards for overhead bins as appliccable for seats, that way they won't break and they won't open. So save people from impact with carry-on (13% reduction in avoidable fatalities), plus either rearward facing seats or 3-point belts (19/18% reduction). Total 32 % from those two mods).

@Koyovis I suspect that 3-point belts would be an easier "sell" to the general public than rearward-facing seats. Most people (at least most people who are likely to fly commercially) are used to those from cars already. Not saying which would be easier or cheaper to implement. Care would obviously need to be taken to ensure passengers can get out of their seat belts quickly in case of an emergency (it would hurt to get entangled in one during a crash, survive with minor injuries only to not be able to get out of your seat due to the seat belt and die from some secondary cause, such as smoke).

Flying in military VC-10s or Tridents back in the '80's, the brace position we were shown had arms crossed across the chest, not in front of the face.

Sorry, too long for a comment, but wanted to add it anyway.
The main problem is that the safety of a rear-facing seat is not really proven. There is cabin debris, and the fact that slowing down by impact these days is much "harder" than in a prop plane.
The disadvantages however are well known.

People don't like to accelerate backwards. It makes them sick.
People would interpret the backwards seats as "wrong"
People would feel that the airplane is going to crash, just because plans are being made for the "what if". For example, I could totally see someone saying "See there going to crash, if they thought it was safe they wouldn't point the seats that way".
People are often ignorant and trying to teach them that a safer way would usually result in a loss in sales.
Any perceived changes to the seats would make the airline look cheap. "They only put them backwards so they could cram more people in."

Now those downsides are only observations of my own, but it's easy to see how backward-facing seats have a real downside, while their upside is not as strong.
Keep in mind that safety for an airline is a function of making money, not a primary concern. If people die in a crash you don't have to pay as much.

"They only put them backwards so they could cram more people in." - it would be quite revolutionary if that actually worked.

@pipe After reversing the seats to cram more people in, you could reverse them again iteratively.

@pipe all I know is that in my lifetime as my ass gets larger the seats actually get smaller. It would be nice if at least one of them stayed the same size.

"If people die in a crash you don't have to pay as much." Crashes typically cause damage to the airframe, or are caused by damage to the airframe; in the latter case, the damage existing during the flight is typically made worse by the deceleration at the end of the flight. Damage to the airframe, regardless of the exact reason why it is sustained and at what point during the flight it is sustained, is typically expensive to repair. Ergo, all else equal, compared to a normal landing, crashing would seem to be bad for the airline's profit margin.

Yes but dead people don't have life long medical bills to payout.

The question is about lives saved. The safety of rearward facing seats is proven.

Facing rearward overall is much more comfortable than forwards - because braking on landing is much higher g than acceleration on takeoff. Facing backwards means you don't have that really uncomfortable bit where you are pushing against the seat in front once the brakes are applied. I know I'd always prefer the rear facing seats.

What do you mean by this?  If people die in a crash you don't have to pay as much  I am puzzled by this assertion.

There'e never one type of impact.
Aircraft on impact can spin, jolt, stop, flip, and list in all sorts of directions.
There really isn't a safer or not safe option.
I'd like to think most aircraft that sadly impact of so forwards, and into something, but I don't have data for that.
I personally choose rear facing as I feel safer (I've seen videos of the safety dummy tests for the seats i travel in and rear facing have less impact on the body in a forward face on impact at high G).
I find the forward brace position uncomfortable and worry about strain on my lower torso. Rear facing I feel supported by the seat back.
The other thing to consider now is those sideways an diagonal seats. They wouldn't be allowed if deemed unsafe so I guess I'd just advise to take whichever seat you are most comfy in, as you're more likely to land that crash!


text:
How do commercial flights get overbooked?
The other week my flight was overbooked and so the airline looked for volunteers to take another flight the following morning. I can't stop thinking about this since the number of seats available and sold should've been known to the airline for quite some time.
Someone claimed that airlines expect a number of passengers to rebook or not show up and thus with purpose overbook sometimes. Can this really be true? Or is it a data consistency thing where availability for ticket booking is prioritized? Or was it perhaps due to an unplanned change of aircraft model? I'm certain there might be other answers and that reasons may vary from occasion to occasion and different airlines, but please enlighten me here!

Someone claimed that airlines expect a number of passengers to rebook or not show up and thus with purpose overbook sometimes. Can this really be true? - yes.  That's exactly why they do it.

If this is closed as off-topic, it should probably be migrated to [travel.se].

Purposely overbook sometimes? That last word is the only part that's wrong.

with all due respect, this was a question regarding airline operations and I can't disagree more with your moderation here. Because this airline operation topic concerns passengers you feel this should be moved to travel? Then make that clear in the tag description. I was interested in the operational cause. I won't accept any answer with the question being flagged OT

I agree.  The tag was Airline Operations, and the question was clearly about how airlines operate.  How is this off-topic?  Maybe it would help us users understand if there was a better explanation given.  Or up/down vote buttons for moderator actions?

Airlines have lots of historical data for any given flight segment that they can use to give statistical probabilities of people missing flights due to various factors such as misconnection or just not showing up.  An airline is a profit driven corporation and to maximize revenue they want full airplanes.
If they know for a flight that 5% +/- 2% of passengers fail to board they can pretty safely sell around 103% of the seats to maximize passengers and minimize risk of overboarding.  Statistics are averages, however, and if you sell 103% of your capacity it is entirely possible for a given instance of that flight to be overbooked.
In most cases of overbooking, volunteers can be found that will give up seats if the plane becomes full.  US airlines generally top out at around $800 compensation for volunteers, so if you are willing to do so it can be advantageous to wait for them to get desperate for volunteers.  If no volunteers can be found it is possible to be involuntarily bumped off the flight and you should consult with the specific airline for any rights you may have in that case.


text:
Are there any active aircraft remaining that require a 5 person flight crew?
Are there any active aircraft still in service (bar for historical air shows etc) that still require the following roles?

2 pilots
1 Flight Engineer
1 Navigator
1 Radio Operator

The radio operator position hasn't existed in decades, the technology has moved on too far for it to be necessary.

Are you asking specifically for exactly a 5 person crew?  Or "at least 5" crew count?

The An-124 is listed as requiring a crew of 8: pilot, copilot, navigator, chief flight engineer, electrical flight engineer, radio operator, and two loadmasters. This includes all of the roles on your list, including 2 flight engineers. The An-225, if it flies again, appears to have a similar crew requirement.
Other aircraft that are still in service with the 5 crew positions you listed:

An-26
An-30
Il-76
Mi-26

As noted in the comments, modern technology has made it possible for even large aircraft to be operated by just 2 pilots. The radio operator position is particularly hard to find still being used. You're looking for something both older and complex, maybe not even not technologically advanced for its time, that hasn't been made completely obsolete. That seems to be a specialty of Russian aircraft.
There are also military aircraft with crews of at least 5 but the extras are usually combat positions not directly related to the positions you list.
Notable mentions:
The E-3 has a flight crew of 4 but when flying missions some of the additional crew could be considered radio operators.
The P-3A mission crew of 12 includes the 5 positions listed. US Navy P-3Cs combined the navigation and radio functions. There are still some countries flying the P-3A. Other aircraft with similar missions from that era may include similar positions.

The An-225 is never going to fly again sadly, unless someone builds a new one from scratch (which doesn't seem likely).

@dkwarr87  Actually, there is a small chance that it will - there is a "half-built" hull already, though last touched quite some time ago, I believe.

@dkwarr87 last year it was announced that there are plans to replace the An-225, probably by completing the second airframe that is still unfinished. There seem to be varying expectations of cost and schedule so we'll see if anything comes of it.

Older C-130's, the H models (with 4-blade props, distinct from the J-model Herks with 6 or 8-blade props) nominally have a crew of 5: 2 pilots, a flight engineer, a navigator, and a loadmaster. That will go up to 6, with 2 loadmasters, for airdrop or certain other missions. The aircraft can also be flown with just 4 - no navigator, but that's limited to fairly short flights, typically "beating up the pattern" for multiple approaches and landings for the pilots.
Once those airplanes are retired and the whole fleet is J-models, the engineers & the navigators will no longer be found in the C-130 community, other than a few special mission birds (i.e. gunships) that will still have navs in their mission crew.

I'm assuming you could also fly the aircraft without a loadmaster if it was empty?

What makes loadmasters part of the crew in a way that cabin attendants aren't? Because otherwise the A380 is going to beat anything, with at least 23 crew when including flight attendants.

@user1937198 good point - the live-load tends to move around and do dumb stuff by choice, compared to cargo which should just stay put.   Cabin crew are vital.

Loadmasters had some duties that you couldn't avoid without rewriting various procedures. The LM closed the last door, because he was outside for engine start. And for certain landing gear malfunctions, the LM had a role nobody else was trained to do.  So while you could rewrite things to get the plane in the air with no LM, you'd be open to some bad scenarios.

The Tupolev Tu-95MS strategic bomber is still in service with the Russian Aerospace Forces. It usually carries a crew of 6 to 7; pilot, co-pilot, flight engineer, communications system operator, navigator, tail gunner, plus sometimes another navigator. Source: Wikipedia.

On Tu-95MSM tail guns are gone

@Severin Papadeux: Excellent! This brings the crew to 5, which is what OP was asking for.

B-52 has a crew of 5: pilot, copilot, navigator, weapon officer, and Electronic Warfare officer.

You don't need the weapons or EWO positions if you're just flying from one place to another.

You would have to have a pilot and a copilot as some controls can only be reached by one or the the other.

@Jim: yes sure, pilot and copilot. It would be nice to know if a B-52 was ever flown by one pilot only.

I worked on B52Gs. on those aircraft the copilots had the fuel, starters, generators, and landing gear, but the landing gear could be reached by the pilot as it was just over the throttle quadrant. The pilot had the hydraulics, lighting, anti ice. On long missions they would have an extra pilot sitting in the instructors seat just behind the throttle quadrant to give the pilot and copilot a break. The G models did not have a bunk, but if you were not too tall you could stretch out on the floor behind the IP seat.

@Jim: thanks

Older Il-76, crew of 5, though loadmaster is included into the crew, I believe


text:
Would building an airport at 30,000 ft save money?
So... this would obviously cost a ton of money initially, but long-term, I believe the savings both environmentally and financially would be returned many times over.
If a building can already be made to reach the clouds (as below), and that is a tall, slim building, then it would theoretically be possible to do the same thing but with a huge, wide building (i.e. an airport). Now, I'm not saying it could reach 30,000ft (or could it?!), but even if an airport could be built (complete with runway) at 10,000ft, the massive savings in takeoff would be returned on the initial outlay.
Aircraft would simply roll off the runway and would already be at cruise, or only have a small, gentle climb to cruise. It would probably look like an aircraft carrier, just... really high.
Passengers, staff and parts could all be transported up and down via elevators (yes, they would need oxygen masks between the airport and the aircraft and the airport would need its own oxygen supply and pressurization).
Is this possible? If yes, why hasn't it been done?

You also haven't considered how to get people on/off the plane - depressurizing the plane will create significant problems of its own.

I'm good reopening this. While the proposal is wildly unrealistic (even at 10k, let alone 30k), the reasons why it is are a good discussion that touches at least as much on aviation as on Civil Engineering. You can still have good discussions about how "this" is an unworkable plan; I think this question is better than it's getting credit for.

@RalphJ But this stack isnt about civil engineering. Even civil engineering for airports isnt really about aviation. We have a list of topics in the [help] which I'm open to changing but perhaps this is a better discussion for a meta post than the comments under a question.

@Jamiec Also, when a question has +11 votes on answers, it implies people were interested in reading the answers, which would not have been possible without the question.

@Cloud lots of votes on questions/answers which are a bad fit for the site (IMO, like this one) usually means its made it to HNQ, and answers have received drive-by votes from well meaning visitors, but ones who are not up on our on-topic-ness.

In actual fact this  question hasn't made it to HNQ, but as has been explained to you in the past @Cloud, an answer can be good (upvoted) even on a poor question (downvoted).

@Jamiec  My point was that this question is sufficiently about aviation to be worthwhile: why aren't high runways a thing? (Clearly, they aren't - nobody has built them as described, but why aren't the advantages of being immediately at higher altitude persuasive?)  Sure, there are C.E. aspects (and Physics, and probably others too) to the question, and some answers, but at heart I see it as being about aviation & why things are "this" way rather than "that" way. JM2C, and I'm good with letting the voting process work.

If this question stuck to the aviation aspect of the effect of take off and landing at high altitude I'd back reopening it. But it asks all kinds of other non-aviation related things (such as economics, civil engineering, passenger-side etc).

"I think this question is better than it's getting credit for."  I couldn't disagree more.  Not to be rude to the OP, but I personally think this rates in top of the worst ideas category.  Airplanes experience greatly degraded takeoff performance at high elevation airports in the 5-10K' range, I can't hardly imagine how long the take-off roll would be at 30K'.  Discounting entirely the pressure and temperature issues, transporting pax, and other things mentioned, just how would you even build an Everest high field with miles and miles of runway that can stand 120mph winds?  What material?!

@MichaelHall Why would you need lift to takeoff? You're 30,00 feet in the air. Simply roll off the runway (in a stall condition) and use a few thousand feet to recover and gain airspeed. Landing would still need to be at high speeds, but takeoffs could be the same as ground takeoffs, speed-wise.

While this isn't a good question, it can be edited to be aviation-related. Just remove the building problem part, and assume it's a given.

I think this is a good question! Imaginative and worth the time to answer. Sorry they downvoted you.

@Justintimeforfun Thanks! That's why I have stopped using all SE sites. Bad environment, negative attitude of moderators, etc.

@Cloud no room for dreamers with our unanswerable questions. Don't give up. Is there another Q and A place like SE?

@Cloud Also, I made a few changes. Building a runway would be possible on a land at 30,000 ft.

The Burj Khalifa in Dubai is currently the tallest building in the world.
It is 828m (2717ft) tall and cost about 1.5 billion US dollars to build.
Putting all technical and logistical difficulties aside, let us assume one can build
a similar structure to 10,000ft, so about a factor 4 taller. Since building
cost probably scales with volume, this would make our new building
considerably more expensive:
$$ 4^3 \times \$1.5 \, \mathrm{billion} = \$96 \, \mathrm{billion} $$
And that is just one small tip at 10,000ft. A whole runway would require a
much larger structure. The runway itself would have to be considerably
longer at these altitudes, because the air is thinner. This would easily
put the total cost in the trillion dollar range.
How much fuel is actually saved by this?
This
answer says a Boeing 737-800 requires about 2300kg of fuel for takeoff and
climb. Even a very generous assumption of saving 2t of fuel per takeoff from
this airport would only save $1200 at current jet fuel prices
per takeoff. London Heathrow had about 475000 movements in 2018
(takeoffs and landings). Assuming similar traffic, it would take
$$ \frac{\$1 \, \mathrm{trillion}}{\$1200} \times \frac{2 \, \mathrm{yr}}{475 \, 000} \approx 3500 \, \mathrm{yr} $$
to pay for the investment (and this is a very generous calculation).
Going to 30,000ft would make this even more expensive by at least a factor $ 3^3 = 27 $.

Plus certification cost, as aircraft are normally only certified for takeoff up to 8000ft or so, depending on specs, so each aircraft type requires (re-)certification for this single/unique „air“-port...

How long would it take to recoup the billions (tens of billions?) of dollars it would cost to build a tower even as little as 10,000 feet high?  Millions of flights, if it saves a thousand dollars per flight -- and I strongly doubt the saving would be anything like that large.
Worse, if you could build to 30,000 feet (and for a tower a couple miles long, I don't doubt it's possible) passengers wouldn't just need oxygen masks to get from terminal to aircraft, they'd need a pressurized boarding ramp.  Apron personnel would need full pressure suits, tugs and luggage tractors would need a complete redesign -- and operating costs of the terminals would likely increase by much more than the saving in fuel (not to mention the runway would have to be much longer, as stall speed at 30,000 is significantly higher -- in ground speed -- than near sea level.
Generally, airports are better near sea level, because the higher the altitude of the field, the faster the aircraft have to land, the longer it takes (in time, and moreso in distance) to accelerate to the increased rotation speed.  Far from saving money, flying out of, say Denver as opposed to, say Miami costs more -- because the runways have to be longer (and if it's hot in Denver, they need to be longer yet).
tl;dr Being able to lift off slow and land slow are more important that the small saving in climbing through 10,000.

Why would you need lift to takeoff? You're 30,00 feet in the air. Simply roll off the runway (in a stall condition) and use a few thousand feet to recover and gain airspeed. Landing would still need to be at high speeds, but takeoffs could be the same as ground takeoffs, speed-wise.

@Cloud Oh, that's going to go over well with passengers right after they see apron crew working in pressure suits.  "But your tickets are almost three dollars cheaper if you fly from Superhigh!"

For that matter, can you even start a high-bypass turbofan at 30,000 feet and -20 C?

If you couldn't, what would be the point in engine restart checklists? (Which are run through at cruise, if that's when an engine fails)

@Cloud That's a hot engine that's been shut down for no more than a few minutes after running at cruise power.  Very different from starting an engine that's been sitting on the ground for an hour or more while you unload, load, and refuel the aircraft.

All passengers would also need to wear pressurised suits with individual oxygen tanks for takeoff and landing, in case there's a need to evacuate during takeoff/landing.

The potential for savings is not there.
Most of the fuel spent on climbing is not wasted. It's recouped back in the descent, which is performed at very low thrust. An aircraft's combination of kinetic and potential energy is a perfect energy storage system.
The only major losses of energy added by the climb-descent cycle are those from the added friction drag of passing through denser air. These are fairly small compared to the overall flight's fuel consumption.
Starting at 30,000 ft, you still have a takeoff to perform. The airplane will need to accelerate, to a higher velocity than normal, to stay in the air and be able to perform a go-around if necessary. At the end of the trip, the plane still needs to bleed airspeed, although less of it.
Meanwhile, there's lot of reasons not to place an airport at 30,000 ft. High altitude airports are much more dangerous compared to normal ones. High winds, high approach speeds, limited runway length combine to that. Part or all of the money saved in fuel would be compensated for by the losses from the increased crash rate.
To compensate for the increased stall speeds, high-altitude airports need longer runways than usual. The currently second-tallest airport also features the longest public runway in the world. High airports also place restrictions on takeoff mass, which means you'll sometimes have to use heavier aircraft at reduced load (e.g. A330 instead of A321) to meet your range and capacity requirements, which wastes much more fuel than low-altitude climb does.
So even if the height itself cost nothing, you wouldn't want to build your airport as high as possible. For a combination of safety, efficiency and cost reasons, you'd still want it on the ground. The extra energy cost from drag would be more than compensated for by the ability to take off at full weight, meaning that routes can be served by smaller,  and often with reduced takeoff thrust.
About the only serious reason to place the airport very high would be for noise abatement.